Conventional white light sources include such devices as incandescent and fluorescent lamps. Such devices have several undesirable characteristics, including size, power consumption, limited operating life, among others. Alternative white light sources having improved characteristics are desired.
Semiconductor devices have been investigated as alternative sources of white light with a goal of finding white light sources that are both power efficient and cost efficient. Light emitting diodes (LEDs) are compact and emit light of clear color with high efficiency. Because they are solid-state elements, LEDs have long operating lifetimes, have good initial drive characteristics, exhibit good vibration resistance, and withstand repetitive ON/OFF operations. Thus, they have been widely used in such applications as various indicators and various light sources. In many applications, LEDs can provide suitable sources of light with low power consumption.
Conventional LEDs typically have a narrow emission spectrum (generate monochromatic light), and thus do not have a broad emission spectrum necessary to provide white light. Recently, devices using a combination of LEDs to provide three light components of red, green, and blue (R, G and B) have been used to provide white light. Large screen LED displays using these light emitting diodes have been put into use. Strategies for diffusing and mixing the light emitted by the LEDs become important in such devices to generate white light of the desired tone, luminance and other factors of the light emission of such devices. Also, combining three diodes (R, G and B) to provide white light requires a larger package than a single diode. Recently developed devices incorporate multiple light-generating active regions on a single semiconductor chip, where the multiple active regions each emit in a distinct wavelength region such that the combination of active regions cover the visible spectrum to emit white light.
Another typical approach for generating white light combines UV or blue light emitting diodes (LEDs) with luminescent materials (such as phosphors) that down-convert the LED emission to longer wavelength light. In such devices, the UV or blue light emission from an active region of a light emitting diode activates (excites) a phosphor composition that is positioned to receive the LED emitted light. In turn, the excited phosphor composition emits light at a longer wavelength. The net result is a light emitting device that emits light having a plurality of wavelengths over the visible spectrum. An appropriate combination of different wavelengths over the visible spectrum can be perceived as white light by the human eye. The composition of the phosphor is typically adjusted to change the color balance of the emitted light. In some cases, the phosphor composition contains more than one phosphor, wherein each phosphor emits at a different wavelength.
For example, U.S. Pat. Nos. 5,813,753 and 5,998,925 disclose light emitting devices in which a blue LED disposed in a reflective cup is surrounded by a phosphor composition. The blue LED emits blue light, a portion of which excites the phosphors in the phosphor composition. The phosphors are selected such that they emit red and green light upon excitation. The device will typically emit a combination of blue light (unabsorbed emission from the LED), and red light and green light (from the phosphors). The combination of wavelengths of light can be perceived by the human eye as white. The phosphor typically ages over the lifetime of the device, changing the efficiency with which the LED emission is converted to higher wavelength. Thus the output light characteristic of the device changes over the lifetime of the device, especially where multiple phosphors are used.
Thus, a white light emitting device is desired that is relatively small, light weight, has a long effective operating life, is power efficient, and is economical.